Many disease states are characterized by uncontrolled proliferation and differentiation of cells. These disease states encompass a variety of cell types and maladies such as cancer, atherosclerosis, and restenosis. In many such disease states, kinases, important cellular enzymes that perform essential functions by regulating cell division and proliferation, appear to play a decisive role.
The molecular mechanisms and signaling pathways that regulate cell proliferation and survival are receiving considerable attention as potential targets for anticancer strategies. Recently, there has been a notable increase in efforts directed at targeting the MAPK pathway, which integrates a wide array of proliferative signals initiated by receptor tyrosine kinases (RTKs) and G protein-coupled receptors.
The MAPK signal cascade includes the G protein Ras working upstream of a core module consisting of 3 kinases: Raf, MEK1/2, and ERK1/2. Raf phosphorylates and thus activates MEK1/2, which in turn ultimately leads to the activation of ERK1/2. Raf kinase has long been considered an attractive target for drug discovery due to its importance as a potential checkpoint for cancer-related signal transduction (Strumberg and Seeber, Onkologie, 2005, 28: 101-107; Beeram et al., J. Clin. Oncol. 2005, 23: 6771-6790). The importance of the MAPK signaling cascade for the proliferation and survival of tumor cells recently increased with the discovery of activating B-Raf mutations in human tumors. Activating Raf mutations have been identified in melanoma, thyroid, colon, and other cancers (Strumberg and Seeber, Onkologie, 2005, 28: 101-107; Bollag et al., Current Opinion in Investigational Drugs, 2003, 4:1436-1441).
Therefore, in addition to a role in controlling tumors with Ras mutations and activated growth factor receptors, inhibitors of Raf kinase may harbor therapeutic potential in tumors carrying a B-Raf oncogene (Sharma et al., Cancer Res. 2005, 65: 2412-2421).
The mammalian Raf serine/threonine kinase family consists of three 68- to 74-kd proteins termed A-Raf, B-Raf, and C-Raf (Raf-1), which share highly conserved amino-terminal regulatory regions and catalytic domains at the carboxyl terminus. Raf proteins are normally cytosolic but they are recruited to the plasma membrane by the small G-protein Ras. Recruitment by Ras is an essential step for Raf activation by growth factors, cytokines, and hormones. At the membrane, Raf activation occurs through a highly complex process involving conformation changes, binding to other proteins, binding to lipids, and phosphorylation and dephosphorylation of some residues.
A variety of agents have been discovered to interfere with Raf kinase, including antisense oligonucleotides and small molecules. These inhibitors prevent the expression of Raf protein, block Ras/Raf interaction, or obstruct its kinase activity. Down regulation of B-Raf activity by siRNA or through the kinase inhibitor BAY-43-9006 leads to inhibition of the growth of melanoma cells and siRNA-mediated reduction of B-Raf led to decreased tumorigenic potential of 1205 Lu cells. Raf inhibitors that are currently undergoing clinical evaluation show promising signs of anti-cancer efficacy with a very tolerable safety profile. Clinically most advanced is the Raf inhibitor BAY 43-9006, which has recently been approved by the FDA for treatment of metastatic renal cell carcinoma with additional phase III clinical testing for treatment of other cancers.
Despite the progress that has been made, the search continues for low molecular weight compounds that are useful for treating a wide variety of tumors and other proliferative disorders including restenosis, angiogenesis, diabetic retinopathy, psoriasis, surgical adhesions, macular degeneration, and atherosclerosis. Thus, a strong need exists to provide compositions, pharmaceuticals and/or medicaments with anti-proliferative activity. Such compositions, pharmaceuticals and/or medicaments may possess not only strong activity, but also exert diminished side effects in comparison to other anti-proliferative agents. Furthermore, the spectrum of tumors responsive to treatment with such compositions, pharmaceuticals and/or medicaments may be broad. Active ingredients of this type may be suitable in the mentioned indication as single agent, and/or in combination therapy, be it in connection with other therapeutic agents, with radiation, with operative/surgical procedures, heat treatment or any other treatment known in the mentioned indications.